Solid state imaging device, method of outputting imaging signal and electronic device

ABSTRACT

There is provided a solid state imaging device including a first analog-to-digital converting unit that compares a level of an analog reference signal with a first inclination with a level of an analog pixel signal output from a pixel array unit, and converts the analog pixel signal into a first digital pixel signal, a second analog-to-digital converting unit that compares a level of an analog reference signal with a second inclination that is different in inclination from the analog reference signal with the first inclination with a level of the analog pixel signal, and converts the analog pixel signal into a second digital pixel signal, and a digital compositing unit that composites the first digital pixel signal with the second digital pixel signal, and outputs a composite result.

BACKGROUND

The present technology relates to a solid state imaging device, a methodof outputting an imaging signal, and an electronic device, and moreparticularly, to a CMOS image sensor.

In the past, techniques of enlarging a dynamic range by finelyquantizing a dark portion of a pixel signal of a solid state imagingdevice with a high gain and roughly quantizing a bright portion with alow gain have been known. For example, JP 2008-124842 A discloses atechnique of generating an analog reference signal with a smallinclination and an analog reference signal with a large inclination bytime division, comparing a level of each analog reference signal with alevel of an analog pixel signal, and obtaining a digital pixel signal.

SUMMARY

In the technique discussed in JP 2008-124842 A, two line memories fortemporarily storing the digital pixel signals generated using the twoanalog reference signals are necessary, and thus the circuit sizeincreases. Further, since the time division is employed, theabove-mentioned technique is not optimal for increasing the speed of ADconversion (analog-to-digital conversion).

The present technology is directed to reducing the circuit size andachieving a high-speed operation and low power consumption.

According to an embodiment of the present disclosure, there is provideda solid state imaging device including a first analog-to-digitalconverting unit that compares a level of an analog reference signal witha first inclination with a level of an analog pixel signal output from apixel array unit, and converts the analog pixel signal into a firstdigital pixel signal, a second analog-to-digital converting unit thatcompares a level of an analog reference signal with a second inclinationthat is different in inclination from the analog reference signal withthe first inclination with a level of the analog pixel signal, andconverts the analog pixel signal into a second digital pixel signal, anda digital compositing unit that composites the first digital pixelsignal with the second digital pixel signal, and outputs a compositeresult.

In the present technology, the first analog-to-digital converting unitcompares a level of an analog reference signal with a first inclinationwith a level of an analog pixel signal output from a pixel array unit,and converts the analog pixel signal into a first digital pixel signal.Further, the second analog-to-digital converting unit compares a levelof an analog reference signal with a second inclination with a level ofthe analog pixel signal, and converts the analog pixel signal into asecond digital pixel signal. Here, the analog reference signal with thesecond inclination is different in inclination from the analog referencesignal with the first inclination. Further, the digital compositing unitcomposites the first digital pixel signal with the second digital pixelsignal, and outputs a composite result.

As described above, in the present technology, the first digital pixelsignal and the second digital pixel signal are simultaneously obtainedby the first analog-to-digital converting unit and the secondanalog-to-digital converting unit. Thus, since a line memory thattemporarily stores the first digital pixel signal and the second digitalpixel signal is not necessary, the circuit size can be reduced, and theproduct cost can be reduced. Further, the first digital pixel signal andthe second digital pixel signal are not obtained by time division, an ADconversion time can be reduced, and a high-speed operation and low powerconsumption can be achieved.

Further, the digital compositing unit may include a determining unitthat, of the first digital pixel signal and the second digital pixelsignal, regards a digital pixel signal converted using an analogreference signal with a small inclination as one digital pixel signal,regards a digital pixel signal converted using an analog referencesignal with a large inclination as the other digital pixel signal,compares the one digital pixel signal with a predetermined thresholdvalue to determine a magnitude relation, and obtains a selection signal,a selecting unit that extracts the one digital pixel signal when it isdetermined that the one digital pixel signal is less than thepredetermined threshold value based on the selection signal, andextracts the other digital pixel signal when it is determined that theone digital pixel signal is equal to or more than the predeterminedthreshold value, and a combining unit that combines the extracteddigital pixel signal with the selection signal, and outputs a combinedsignal. In this case, it is possible to suppress output bandwidth and todecrease processing load in an external circuit, comparing to the casewhere both of the first digital pixel signal and the second digitalpixel signal are to be output. The solid state imaging device mayfurther include a signal processing unit that receives an output signalof the digital compositing unit, and performs magnification correctionon the digital pixel signal based on the selection signal combined withthe digital pixel signal.

Further, the digital compositing unit may combine the first digitalpixel signal and the second digital pixel signal to be composited into asignal with a large bit width and outputs a composite result, orinterleaves and composites the first digital pixel signal and the seconddigital pixel signal, and output a composite result. In this case, thesolid state imaging device may further include a signal processing unitthat processes an output signal of the digital compositing unit. Thesignal processing unit may include a determining unit that, of the firstdigital pixel signal and the second digital pixel signal, regards adigital pixel signal converted using an analog reference signal with asmall inclination as one digital pixel signal, regards a digital pixelsignal converted using an analog reference signal with a largeinclination as the other digital pixel signal, compares the one digitalpixel signal with a predetermined threshold value to determine amagnitude relation, and obtains a selection signal, a selecting unitthat extracts the one digital pixel signal when it is determined thatthe one digital pixel signal is less than the predetermined thresholdvalue based on the selection signal, and extracts the other digitalpixel signal when it is determined that the one digital pixel signal isequal to or more than the predetermined threshold value, and a signalprocessing unit that performs magnification correction on the extracteddigital pixel signal based on the selection signal.

Further, the digital compositing unit may further include an estimatingunit that calculates correction information used to accurately performmagnification correction.

Further, the first inclination and the second inclination may each be aninclination obtained based on a noise characteristic of the pixelsignal. Such inclination enables the reference signal generator to beconfigured with a simple integrator and simply implemented.

Further, the analog-to-digital converting unit may transition to a nextoperation when pulses applied to a counter that counts a conversion timein the analog-to-digital converting unit reach a predetermined number.Thus, the operation can be performed at a high speed, and the powerconsumption can be reduced.

The first and the second inclination, and a predetermined number in theanalog-to-digital converting are given, for example, where a standarddeviation of noise when a light quantity is 0 is represented by σ(electron), a number of saturated electrons of a pixel is represented byVwhite, and parameters are represented by Ka and Kb, by Vthresh causingTover of the following Formula (1) to be minimum, the first inclinationis given as ΔSa, the second inclination is given as ΔSb, and thepredetermined number in the analog-to-digital converting unit is givenas Tover.

$\begin{matrix}{{{Tover}:={\max\left( {{{V_{thresh}/\Delta}\;{Sa}},{{V_{white}/\Delta}\;{Sb}}} \right)}}\begin{pmatrix}{{\Delta\;{Sa}} = {{Ka}*\sigma}} \\{{\Delta\;{Sb}} = {{Kb}*\sqrt{\sigma^{2} + {Vthresh}}}}\end{pmatrix}} & (1)\end{matrix}$

Further, the analog reference signal with the first inclination and theanalog reference signal with the second inclination may be in a mutuallyscaled relation. In this case, for example, one of the analog referencesignal with the first inclination and the analog reference signal withthe second inclination may be generated by a reference signal generator,and the other may be obtained in a manner that an analog referencesignal generated by the reference signal generator is amplified by anamplifier or attenuated by an attenuator. Thus, the product cost can bereduced.

Further, in the present technology, for example, the pixel array unit isa pixel array unit in which pixels having greatly different sensitivityof light are present together. For example, the pixel array unit mayinclude both a pixel having sensitivity to visible light and a pixelhaving sensitivity to invisible light. Further, for example, the pixelarray unit may include both a pixel having sensitivity in a narrow bandand a pixel having sensitivity in a broad band. In this pixel arrayunit, a digital pixel signal having little noise and a large dynamicrange can be obtained.

Further, in the present technology, the solid state imaging device mayfurther include a multiplexer that selectively supplies theanalog-to-digital converting unit with pixel signals of a plurality ofcolumns of the pixel array unit. In this case, the number ofanalog-to-digital converting units can be reduced, and even when amounting area of a semiconductor substrate is narrow, mounting can beeasily performed.

Further, in the present technology, the first analog-to-digitalconverting unit and the second analog-to-digital converting unit may bearranged to be opposite to each other with the pixel array unitinterposed therebetween. Thus, the first analog-to-digital convertingunit and the second analog-to-digital converting unit that operate inparallel can be efficiently arranged on the semiconductor substrate.

According to the embodiments of the present technology, it is possibleto reduce the circuit size and achieve a high-speed operation and lowpower consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a configuration example of a CMOSimage sensor according to an embodiment;

FIG. 2 is a diagram illustrating an example of a potential change of ananalog reference signal Vref-A and an analog reference signal Vref-Bwhich are in a mutually scaled relation;

FIG. 3 is a diagram illustrating waveform examples of a plurality ofanalog reference signals having different inclinations;

FIG. 4 is a diagram illustrating a relation between Vthresh and Toverwhen inclinations ΔSa and ΔSb of an analog reference signal areobtained;

FIG. 5 is a block diagram illustrating a first configuration example ofa digital compositing unit;

FIG. 6 is a block diagram illustrating a second configuration example ofa digital compositing unit;

FIG. 7 is a block diagram illustrating a third configuration example ofa digital compositing unit;

FIG. 8 is a block diagram illustrating a signal processing unit thatreceives an output of a CMOS image sensor;

FIG. 9 is a flowchart illustrating a signal processing example of asignal processing unit;

FIG. 10 is a flowchart illustrating another signal processing example ofa signal processing unit;

FIG. 11 is a diagram illustrating an example of a hardware configurationof an image display device;

FIG. 12 is a diagram illustrating a detailed configuration example of aCMOS image sensor;

FIG. 13 is a diagram illustrating a configuration example of a unitpixel;

FIG. 14 is a diagram illustrating a configuration of a comparator;

FIG. 15 is a flowchart illustrating the flow of an AD conversionoperation controlled by a timing control circuit;

FIG. 16 is a waveform chart of an example (when input brightness is lowand Vx is small) of an AD conversion operation obtained by an operationof a timing control circuit;

FIG. 17 is a waveform chart of an example (when input brightness is highand Vx is large) of an AD conversion operation obtained by an operationof a timing control circuit;

FIG. 18 is a diagram illustrating an example in which input brightnessis low, Vx is small, and both of an analog reference signal Vref-A witha small inclination and an analog reference signal Vref-B with a largeinclination intersect with an analog pixel signal Vx;

FIG. 19 is a diagram illustrating an example in which input brightnessis high, Vx is large, and an analog reference signal Vref-B with a largeinclination intersects with an analog pixel signal Vx;

FIG. 20 is a diagram illustrating waveforms of an analog referencesignal Vref-A and an analog reference signal Vref-B in the case of aCMOS image sensor of a column AD type;

FIG. 21 is a diagram illustrating a waveform chart generated by a timingcontrol circuit that controls an operation of a CMOS image sensor;

FIG. 22 is a diagram for describing conciseness of an AD converter of acolumn AD type according to the present technology;

FIG. 23 is a diagram for describing complexity of an AD converter of acolumn AD type according to a related art;

FIG. 24 is a diagram of a color array example of a pixel array unit;

FIG. 25 is a diagram illustrating an example in which pixels thatgreatly differ in sensitivity of light such as visible light andinvisible light are present together as a color array of a pixel arrayunit;

FIG. 26 is a diagram illustrating an example in which pixels whichgreatly differ in sensitivity of light such as color of a narrow bandand color of a broad band are present together as a color array of apixel array unit;

FIG. 27 is a block diagram illustrating another configuration example ofa digital compositing unit further including an estimating unit; and

FIG. 28 is a diagram illustrating an example in which both of twoanalog-to-digital converters are accurate and an example in which atleast one of the two analog-to-digital converters is inaccurate.

DETAILED DESCRIPTION OF THE EMBODIMENT(S)

Hereinafter, preferred embodiments of the present disclosure will bedescribed in detail with reference to the appended drawings. Note that,in this specification and the appended drawings, structural elementsthat have substantially the same function and structure are denoted withthe same reference numerals, and repeated explanation of thesestructural elements is omitted.

Hereinafter, a mode for embodying the disclosure (hereinafter referredto as an “embodiment”) will be described. The description will proceedin the following order.

1. Embodiment

2. Modified Example

<1. Embodiment>

[Configuration Example of CMOS Image Sensor]

FIG. 1 illustrates a configuration example of a CMOS image sensor 100according to an embodiment. The CMOS image sensor 100 is used in variouskinds of electronic devices such as video cameras, digital stillcameras, cameras mounted in a mobile phone, monitoring cameras, camerasfor a personal computer (PC), or the like. Here, the CMOS image sensorrefers to an image sensor manufactured by applying or partially using aCMOS process.

The CMOS image sensor 100 includes a pixel array unit 101, multiplexers102A and 102B, AD (analog-to-digital) converting units 103A and 103B, areference signal generator 105A, an amplifier 105B, and a digitalcompositing unit 108. The AD converting units 103A and 103Bsimultaneously convert the same analog pixel signal output from a unitpixel 11 of the pixel array unit 101 into a digital pixel signal inparallel.

The multiplexers 102A and 102B selectively supply pixel signals of aplurality of columns of the pixel array unit 101 (pixel signals of twocolumns in this example) to the AD converting units 103A and 103B. Sincethe number of the AD converting units 103A and 103B can be reduced usingthe multiplexers 102A and 102B as described above, mounting can beeasily performed even when a mounting area of a semiconductor substrateis narrow. The AD converting units 103A and 103B are arranged to beopposite to each other with the pixel array unit 101 interposedtherebetween. Thus, the AD converting units 103A and 103B that operatein parallel can be efficiently arranged on the semiconductor substrate.In other words, even when a column width is narrow, many AD convertingunits can be arranged by the opposed arrangement, and thus the mountingarea can be reduced. Meanwhile, when a mounting cost need not beconsidered or when a sufficient mounting area is secured, a one-sidedarrangement may be employed. In this case, the digital compositing unit108 is arranged for each column, and thus the data transfer rate can befurther increased.

The AD converting unit 103A finely quantizes an analog pixel signal witha high gain to obtain a digital pixel signal. In other words, the ADconverting unit 103A compares a level of an analog reference signal (aramp signal) with a small inclination generated by the reference signalgenerator 105A with a level of an analog pixel signal, and converts theanalog pixel signal into the digital pixel signal. The AD convertingunit 103A includes an analog comparator 106A and a digital counter 107A.The analog comparator 106A compares a level of an analog referencesignal Vref-A with a small inclination generated by the reference signalgenerator 105A with a level of an analog pixel signal output from thepixel array unit 101.

The digital counter 107A measures a conversion time by counting clockpulses, and measures a level value based on the length of the conversiontime. The digital counter 107A stops its counting operation at a pointin time at which the analog reference signal Vref-A serving as the rampsignal intersects with the analog pixel signal based on the comparisonoutput of the analog comparator 106A. The level value measured by thedigital counter 107A serves as an output of the AD converting unit 103A,that is, the converted digital pixel signal.

Further, the AD converting unit 103B roughly quantizes an analog pixelsignal with a low gain to obtain a digital pixel signal. In other words,the AD converting unit 103B compares a level of an analog referencesignal (ramp signal) with a large inclination generated by the amplifier105B with a level of an analog pixel signal, and converts the analogpixel signal into the digital pixel signal.

The amplifier 105B amplifies the analog reference signal Vref-Agenerated by the reference signal generator 105A of the AD convertingunit 103A, and obtains an analog reference signal Vref-B with a largeinclination. In this case, the analog reference signal Vref-A and theanalog reference signal Vref-B are in a mutually scaled relation. FIG. 2illustrates an example of a potential change of the analog referencesignal Vref-A and the analog reference signal Vref-B which are in themutually scaled relation.

In other words, when the analog reference signal Vref-A and the analogreference signal Vref-B are in the mutually scaled relation, the analogreference signal Vref-B can be obtained by the amplifier 105B asdescribed above. Based on a similar consideration, the analog referencesignal Vref-A may be obtained by generating the analog reference signalVref-B through a reference signal generator similar to the referencesignal generator 105A and then attenuating the analog reference signalVref-B through an attenuator. Of course, both of the analog referencesignals Vref-A and Vref-B can be generated by a reference signalgenerator similar to the reference signal generator 105A.

The AD converting unit 103B includes an analog comparator 106B and adigital counter 107B. The analog comparator 106B compares a level of theanalog reference signal Vref-B with a level of an analog pixel signaloutput from the pixel array unit. The digital counter 107B measures aconversion time by counting clock pulses, and measures a level valuebased on the length of the conversion time. The digital counter 107Bstops its counting operation at a point in time at which the analogreference signal Vref-B serving as the ramp signal intersects with theanalog pixel signal based on the comparison output of the analogcomparator 106B. The level value measured by the digital counter 107Bserves as an output of the AD converting unit 103B, that is, theconverted digital pixel signal.

In this embodiment, the inclinations of the analog reference signalVref-A and the analog reference signal Vref-B may be fixed. As theinclinations are fixed, it is not necessary to adjust ISO sensitivitybefore imaging, prediction control of an imaging parameter according toa scene to be imaged is simplified, and thus it is easy to handle acamera. Further, it is not necessary to implement a circuit forimplementing programmability for adjusting ISO sensitivity, and hardwareis simplified. Further, in this embodiment, when the number of pulsesapplied to the digital counters 107A and 107B in the AD converting units103A and 103B reaches a predetermined number, the AD converting units103A and 103B transition to the next operation. As a result, theoperation speed increases, and the power consumption is reduced.

FIG. 3 illustrates waveforms of the analog reference signal Vref-A andthe analog reference signal Vref-B. Sa and Sb represent the analogreference signals Vref-A and Vref-B, respectively, and a count value ofa horizontal axis corresponds to an analog value of a vertical axis in aone-to-one manner. Referring to FIG. 3, a vertical axis (voltage)represents an analog output voltage of a pixel, and particularly, anaverage voltage in a light-shielding state (that is, a state in which areceived light quantity is 0) is represented by Vblack (=0), and avoltage of a white level representing a maximum received light quantityis represented by Vwhite. Meanwhile, the horizontal axis (time)represents a value obtained by counting a time through a digitalcounter.

An inclination ΔSa of the analog reference signal Vref-A, an inclinationΔSb of the analog reference signal Vref-B, and a time (a count number)Tover taken for AD conversion, and the like which are illustrated inFIG. 3 are obtained based on a noise characteristic of an analog pixelsignal as follows.

First, as constants representing characteristics of a pixel signal to beinput to the AD converting unit, a standard deviation of noise of thepixel signal in the light-shielding state (that is, the received lightquantity 0) is σ in the following Formula (4), and a voltage of a whitelevel representing a maximum received light quantity is Vwhite in thefollowing Formula (3). The constants are estimate values obtained by anarbitrary method.

Further, as constants related to the image quality, a parameter fordeciding the image quality of a dark image portion is Ka in thefollowing Formula (4), and a parameter for deciding the image quality ofa bright image portion is Kb in the following Formula (4). For example,normal values of the parameters Ka and Kb are 1 and 1, respectively.When the values decrease, the image quality is somewhat improved, but atime taken for AD conversion increases. For the sake of simplificationof description, a pixel is assumed to convert n electrons into a voltageof n volts by photoelectric conversion.

In the above setting, when all values in a range of [0,Vwhite] areassigned to Vthresh in Formulas (3) and (4) and Tover in the followingFormula (2) is evaluated, Vthresh causing Tover to be minimum isdecided. Tover is a minimum time (count number) taken for AD conversion.FIG. 4 illustrates a relation between Vthresh and Tover.

When Vthresh causing Tover to be minimum is obtained, ΔSa in Formula (4)representing the inclination of the analog reference signal Vref-A, ΔSbin Formula (4) representing the inclination of the analog referencesignal Vref-B, Ta_over in Formula (3) representing a saturation value ofthe AD converting unit 103A, and Tb_over in Formula (3) representing asaturation value of the AD converting unit 103B are dependently decided.

Tover obtained as described above is a minimum pulse number applied fromthe timing control circuit to the counter of the AD converting unit. Inother words, as the timing control circuit is designed to transition tothe next operation immediately after pulses are applied to the counterof the AD converting unit by Tover times, a useless counting operationcan be prevented, and an operation time can be optimized. In otherwords, the effects by which power consumption is reduced and theoperation speed increases are obtained.

$\begin{matrix}{T_{over}:={\max\left( {T_{a\_ over},T_{b\_ over}} \right)}} & (2) \\\left. \begin{matrix}{T_{b\_ over}:=\frac{V_{white}}{\Delta\; S_{b}}} \\{T_{a\_ over}:=\frac{V_{thresh}}{\Delta\; S_{a}}}\end{matrix} \right\} & (3) \\\left. \begin{matrix}{{\Delta\; S_{a}}:={K_{a} \cdot \sigma}} \\{{\Delta\; S_{b}}:={K_{b} \cdot \sqrt{\sigma^{2} + V_{thresh}}}}\end{matrix} \right\} & (4)\end{matrix}$

The digital compositing unit 108 composites a digital pixel signal Vaobtained by finely quantizing the analog pixel signal with a high gainthrough the AD converting unit 103A with a digital pixel signal Vbobtained by roughly quantizing the analog pixel signal with a low gainthrough the AD converting unit 103B, and outputs the composite result.

The digital pixel signals Va and Vb input to the digital compositingunit 108 are digital signals which have been subjected to a correlateddouble sampling (CDS) process. CDS is a signal processing technique usedto reduce reading noise of a pixel signal. Particularly, by detecting adifference in a voltage level between before and after signal charges ofa pixel flow into a charge-voltage converting circuit through CDS, it ispossible to obtain an output in which thermal noise (kTC noise) isreduced. The following Formula (5) represents a relation between ADconversion results Ta and Tb and the digital pixel signals Va and Vbwhen CDS is performed through an analog circuit (not shown).

$\begin{matrix}\left. \begin{matrix}{V_{a}:={T_{a} - 0}} \\{V_{b}:={T_{b} - 0}}\end{matrix} \right\} & (5)\end{matrix}$

FIG. 5 illustrates a first configuration example of the digitalcompositing unit 108. In the example illustrated in FIG. 5, the digitalcompositing unit 108 includes offset adjusting units 201A and 201B and acombining unit 206. In the example illustrated in FIG. 5, the digitalpixel signals Va and Vb are each 11-bit signals.

Each of the offset adjusting units 201A and 201B receives an 11-bitvalue, performs an offset adjustment for shifting a black level to apredetermined value (for example, 16), rewrites a value of 0 or less to0 and a value of 1023 or more to 1023, and outputs a 10-bit value to thecombining unit 206. Thus, valid data can be output to the outsidethrough a bus with a minimal bit number width.

A general technique of performing statistical processing on signalvalues of optical black pixels (not shown) that are constantlylight-shielded (for example, a method of obtaining an average value ofpixels in an OPB region) is used as a technique of calculating a blacklevel. In other words, each of the offset adjusting units 201A and 201Bperforms an adjustment so that a level of the average value becomes avalue (for example, 16) representing a black level as an output value.For example, when the value representing the black level is set to 16, avalue range of −16 to 1007 is encoded into 10-bit data of 0 to 1023, andoutput with a small negative level.

The following Formula (6) represents an offset adjusting process in theoffset adjusting units 201A and 201B. In other words, the offsetadjusting process is a process of calculating a difference between eachof the digital pixel signals Va and Vb and the black level OPB.

$\begin{matrix}\left. \begin{matrix}{D_{a}:={V_{a} - {OPB}}} \\{D_{b}:={V_{b} - {OPB}}}\end{matrix} \right\} & (6)\end{matrix}$

The combining unit 206 combines the digital pixel signals Va and Vb tobe composited into a signal with a large bit width, and outputs thecomposited signal to an external output line. In the example illustratedin FIG. 5, each of the 11-bit signals Va and Vb is converted into asignal with a 20-bit width through the combining unit 206.

FIG. 6 illustrates a second configuration example of the digitalcompositing unit 108. In the example illustrated in FIG. 6, the digitalcompositing unit 108 includes offset adjusting units 201A and 201B and acombining unit 207. In the example illustrated in FIG. 6, each of thedigital pixel signals Va and Vb is an 11-bit signal. Each of the offsetadjusting units 201A and 201B receives an 11-bit value, performs anoffset adjustment for shifting a black level to a predetermined value(for example, 16), and outputs a 10-bit value to the combining unit 207,similarly to the offset adjusting units 201A and 201B in the digitalcompositing unit 108 illustrated in FIG. 5. The combining unit 207performs an interleaving compositing on the digital pixel signals Va andVb using clocks which are twice as many as input clocks.

FIG. 7 illustrates a third configuration example of the digitalcompositing unit 108. In the example illustrated in FIG. 7, the digitalcompositing unit 108 includes offset adjusting units 201A and 201B, adetermining unit 202, a selecting unit 203, and a combining unit 204. Inthe example illustrated in FIG. 7, each of the digital pixel signals Vaand Vb is an 11-bit signal. Each of the offset adjusting units 201A and201B receives an 11-bit value and performs an offset adjustment forshifting a black level to a predetermined value (for example, 16),similarly to the offset adjusting units 201A and 201B in the digitalcompositing unit 108 illustrated in FIG. 5.

The determining unit 202 compares a digital pixel signal Da with Dthreshserving as a threshold value, determines whether or not the digitalpixel signal Da is less than the threshold value, and obtains a 1-bitselection signal sel. For example, when it is determined that thedigital pixel signal Da is the threshold value or more, the selectionsignal sel has a value of “1,” but when it is determined that thedigital pixel signal Da is less than the threshold value, the selectionsignal sel has a value of “0.”

The threshold value Dthresh is a digital value corresponding to a levelof the level Vthresh of the analog signal of FIG. 3, and is a valueobtained by calculating, for example, the following Formula (7).D _(thresh)=(V _(thresh)−V_(black))/ΔS _(a)  (7)

The following Formula (8) represents a determining process in thedetermining unit 202. In other words, the determining process is aprocess of comparing the digital pixel signal Da of an A system with thethreshold value Dthresh and deciding the value of the selection signalsel.

$\begin{matrix}\left. \begin{matrix}{C_{a}:=\left( {{Da} < {Dthresh}} \right)} \\\left\{ \begin{matrix}{{sel}:=0} & {{if}\mspace{14mu} C_{a}\mspace{14mu}{is}\mspace{14mu}{true}} \\{{sel}:=1} & {otherwise}\end{matrix} \right.\end{matrix} \right\} & (8)\end{matrix}$

The determining process is not limited to the method using the digitalpixel signal Da of the A system but may be a method using the digitalpixel signal Db of a B system, for example. In this case, the selectionsignal sel may be obtained by comparing the digital pixel signal Db ofthe B system with a threshold value having a level equivalent to thethreshold value Ta_over of the A system. The equivalent level refers tothe digital value of the B system corresponding to the digital value ofthe A system in which the analog pixel signal is almost the same level.

Based on the selection signal sel, the selecting unit 203 extracts thedigital pixel signal Da when it is determined that the digital pixelsignal Da is less than the threshold value, and extracts the digitalpixel signal Db when it is determined that the digital pixel signal Dais the threshold value or more. The following Formula (9) represents aselecting process in the selecting unit 203. In other words, theselecting process is a process of selecting either the digital pixelsignal Da of the A system or the digital pixel signal Db of the B systemaccording to the value of the selection signal sel and deciding adigital pixel signal Dc.

$\begin{matrix}\begin{Bmatrix}{D_{c}:=D_{a}} & {{{if}\mspace{14mu}{sel}} = 0} \\{D_{c}:=D_{b}} & {otherwise}\end{Bmatrix} & (9)\end{matrix}$

The combining unit 204 combines the digital pixel signal Dc extracted bythe selecting unit 203 with the selection signal sel into one word (an11-bit signal), and outputs the combined one word to the external outputline.

FIG. 8 illustrates a signal processing unit 1091 that receives an outputof the CMOS image sensor 100. The signal processing unit 1091 processesa signal imaged by the CMOS image sensor 100. When the digitalcompositing unit 108 of the CMOS image sensor 100 employs theconfiguration of FIG. 5 or 6, the signal processing unit 1091 performsthe selecting process and the magnification correcting process and thenperforms general camera signal processing according to a flowchart ofFIG. 9 which will be described later. When the digital compositing unit108 of the CMOS image sensor 100 employs the configuration of FIG. 7,the signal processing unit 1091 performs the magnification correctingprocess and the general camera signal processing according to aflowchart of FIG. 10 which will be described later. The general camerasignal processing refers to a process of converting a signal imaged byperforming the white balance process, the de-mosaic process, the tonemapping process, the gamma correction process, or the like into imagedata.

FIG. 9 is a flowchart representing a signal processing example of thesignal processing unit 1091 when the digital compositing unit 108employs the configuration of FIG. 5 or 6. The imaging signal output fromthe digital compositing unit 108 having the configuration of FIG. 5 or 6includes the digital pixel signal Da (corresponding to A0 to A9 of theoutput data of FIG. 5 or 6) and the digital pixel signal Db(corresponding to B0 to B9 of the output data of FIG. 5 or 6). In stepST11, the value of the digital pixel signal Dc is decided by Formula (9)according to the value of the selection signal sel obtained by Formula(8).

Next, in step ST12, the magnification correcting process is performed oneach pixel signal Dc according to the value of the selection signal selto obtain a value of a pixel signal D. The following Formula (10)represents the magnification correcting process. In other words, themagnification correcting process is a process of multiplying the digitalpixel signal Dc by ΔSa or ΔSb according to the value of the selectionsignal sel and correcting a magnification rate.

$\begin{matrix}\begin{Bmatrix}{D:={{D_{c} \cdot \Delta}\; S_{a}}} & {{{if}\mspace{14mu}{sel}} = 0} \\{D:={{D_{c} \cdot \Delta}\; S_{b}}} & {otherwise}\end{Bmatrix} & (10)\end{matrix}$

After obtaining the pixel signal D on all pixels, in step ST13, thegeneral camera signal processing is performed to obtain image data thatis viewed as a digital photograph by a human being.

FIG. 10 is a flowchart illustrating a signal processing example of thesignal processing unit 1091 when the digital compositing unit 108employs the configuration of FIG. 7. The imaging signal output from thedigital compositing unit 108 having the configuration of FIG. 7 includesthe digital pixel signal Dc (corresponding to C0 to C9 of the outputdata of FIG. 7) and the selection signal sel (corresponding to sel ofthe output data of FIG. 7).

In step ST12, similarly to step ST12 of the flowchart of FIG. 9, themagnification correcting process is performed on each pixel signal Dcaccording to the value of the selection signal sel to decide the valueof the pixel signal D. After obtaining the pixel signal D on all pixels,in step ST13, the general camera signal processing is performed toobtain image data that is viewed as a digital photograph by a humanbeing.

Further, in a transmission path between the image sensor 100 and thesignal processing unit 1091, the amount of information may be furtherreduced using an information compression device and an informationdecompression device and then communicated.

The image sensor 100 and the signal processing unit 1091 may be mountedin a single device or different devices. In other words, the signalprocessing unit 1091 may be mounted in a digital processing processor(DSP), separately from the image sensor 100. The transmission pathbetween the image sensor 100 and the signal processing unit 1091 is notlimited to a direct connection, and may involve a computer or theInternet. In other words, storage transmission may be performed througha storage, a network, and the like.

FIG. 27 illustrates an example in which the digital compositing unit 108is further provided with an estimating unit 401 that causes an analoggain to match a digital gain as accurately as possible. The estimatingunit 401 receives a plurality of digital pixel signals Da and aplurality of digital pixel signals Db, and performs a calculation ofestimating information corresponding to the inclinations of the analogreference signal Vref-A and the analog reference signal Vref-B from thereceived information. The estimated information is output to the signalprocessing unit 1091 of a subsequent stage as correction information402.

The correction information 402 generated by the estimating unit 401 isused to more accurately perform a calculation of correcting amagnification rate through the magnification correcting processperformed by the signal processing unit 1091. The correction information402 is necessary because the reference signal generator 105A and theamplifier 105B are analog circuits, and thus a manufacturing error islikely to occur. For example, even when the analog circuit is designatedsuch that the inclination of the analog reference signal is 10electrons/STEP, an error such as 9.7 electrons/STEP is likely to beincluded, and it is difficult to manufacture an accurateanalog-to-digital converter. If magnification correction is performedunder the assumption of 10 electrons/STEP even when the inclination ofthe analog reference signal is inaccurate, for example, 9electrons/STEP, a problem such as a color change shown in a resultantimage occurs.

FIG. 28 (a) illustrates an example in which both of theanalog-to-digital converters are accurate, and FIG. 28 (b) illustratesan example in which at least one of the two analog-to-digital convertersis inaccurate. In this example, the inclination of the A systemincluding one analog-to-digital converter is assumed to have 1electron/STEP, and the inclination of the B system including the otheranalog-to-digital converter is assumed to have 10 electrons/STEP, thatis, the two analog-to-digital converters are accurate. In other words,the digital value of the A system has a magnification rate 10 timesgreater than the digital value of the B system. For example, the valueof the A system corresponding to 10 or more and less than 11 as thevalue of the B system in FIG. 28 (a) is 100 or more and less than 110.

However, when the two analog-to-digital converters are inaccurate andthe magnification rate in which the digital value of the A system is 9.7times the digital value of the B system is given, the value of the Asystem corresponding to 10 or more and less than 11 as the value of theB system is 97 or more and less than 107 as illustrated in FIG. 28( b).At this time, the magnification rate can be accurately corrected to beequal as in FIG. 28 (a) by converting the value of the A system througha correction value of about 100/97 times. Therefore, when a calculationof correcting the magnification rate through the magnificationcorrecting process performed by the signal processing unit 1091 isperformed using the value of 100/97 times as the correction information402, more accurate correction can be performed.

When the distribution of the value of the digital pixel signal of the Asystem on the value of the digital pixel signal of the B system ismeasured and evaluated as the inaccurate distribution, a correctionvalue for converting the digital pixel signal so that the accuratedistribution corresponding to the value of the B system is made ispreferably obtained as the correction information 402. The method ofobtaining the correction value can be simply described as follows: thevalue of the A system corresponding to the value 10 of the B system isideally 100 as illustrated in FIG. 28 (a), but when the minimum value ofthe value of the A system corresponding to the value 10 of the B systemis actually 97 as illustrated in FIG. 28 (b), 100/97 times is obtainedas the correction value by the ratio of the two values, but the methodof obtaining the correcting value is not limited to this example.

FIG. 11 is a diagram illustrating an example of a hardware configurationof an image display device according to an embodiment. Here, a portabledevice is assumed as a concrete example of the image display device. Theportable device includes an imaging unit 1001, a signal processing unit1005, and a flash ROM 1023 for photography and includes an imageprocessing unit 1021 and a display unit 1031 for image display. Theportable device further includes a processor 1011 for performing overallcontrol, a read only memory (ROM) 1012, and a random access memory (RAM)1013.

The portable device further includes a sensor unit 1006 that acquires anacceleration, an angular speed position, a direction, or the like. Theportable device further includes a data communication unit 1024 and anantenna 1034 for data communication. The portable device furtherincludes an audio input unit 1025 and an audio output unit 1026 foraudio input and output. In addition, the portable device includes anoperating unit 1022 through which a user performs an operation.

The imaging unit 1001 is a photoelectric converter that includes theCMOS image sensor 1004 (corresponding to the CMOS image sensor 100 ofFIG. 8) serving as an imaging element, and forms an image on an imagingplane through an optical block. The imaging unit 1001 includes anoptical filter 1008, an optical lens 1002, an optical diaphragm 1002,and the like as the optical block.

The signal processing unit 1005 (corresponding to the signal processingunit 1091 of FIG. 8) performs a process of converting an image signalimaged by the imaging unit 1001 into image data through the camerasignal processing such as the white balance process, the de-mosaicprocess, the tone mapping process, and the gamma correction. The flashROM 1023 records the image data processed by the signal processing unit1005.

The image processing unit 1021 is a graphic processor with a function ofrendering the image data by performing the coordinate transformation onthe image data. The display unit 1031 displays the image data renderedby the image processing unit 1021, and includes a liquid crystal displaydevice (LCD), for example.

The processor 1011 undertakes processing of the whole portable device,and uses the RAM 1013 as a working area. A program for controlling theimaging unit 1001, a program of the signal processing unit 1005, and aprogram for executing recording control and playback control of imagedata or the like are written in the ROM 1012.

The data communication unit 1024 includes a modem circuit that performsdata communication with another device with a data communicationfunction. The antenna 1034 transmits or receives a radio frequencysignal for data communication by a wireless scheme. The datacommunication may be performed by a wired scheme using a copper wire oran optical fiber. The audio input unit 1025 receives an audio inputprovided from the user, and the audio output unit 1026 supplies the userwith an audio output. The operating unit 1022 is an operation devicethat receives an operation input from the user, and includes a shutterbutton, a numerical key pad, a directional key, a touch panel, and thelike.

[Detailed Configuration Example of CMOS Image Sensor]

FIG. 12 illustrates a detailed configuration of the CMOS image sensor100. The CMOS image sensor 100 includes a pixel array unit 10(corresponding to the pixel array unit 101 of FIG. 1) in which the unitpixels 11 are two-dimensionally arranged in the form of a matrix, aperipheral driving system for driving each pixel 11 of the pixel arrayunit 10, and a signal processing system. A timing control circuit 31, areference signal generator 35, a row scanning circuit 36, a columnscanning circuit 37, a column AD unit 20, a multiplexer 21(corresponding to the multiplexers 102A and 102B of FIG. 1), and adigital compositing unit 39 (corresponding to the digital compositingunit 108 of FIG. 1) are disposed as a peripheral driving system and asignal processing system. The driving system and the signal processingsystem may be integrated on the same semiconductor substrate (chip) asthe pixel array unit 10 or may be mounted on different semiconductorsubstrates and connected to the pixel array unit 10 through multi-chipmounting or stacking mounting.

The unit pixel 11 is a photoelectric converting element, and outputs apixel signal according to a received light quantity. The output of theunit pixel 11 is controlled by a row control line 13 (13-1 to 13-n). Therow scanning circuit 36 performs control such that pixel signals of theunit pixels 11 of any one row are output. The pixel signal output fromthe unit pixel 11 is transferred to a vertical signal line 12 (12-1 and12-2), then selected by a multiplexer (switch) 21, and subjected toanalog-to-digital conversion by the column AD unit 20. The multiplexer21 selectively connects a terminal x to either a terminal a or aterminal b to transfer the pixel signal. The selection is controlled bythe timing control circuit 31.

The column AD unit 20 includes AD converters 29 (corresponding to the ADconverting units 103A and 103B of FIG. 1) which are arranged in columns.The AD converter 29 compares a level of a pixel signal 25 with a levelof a reference signal 32 through a comparator 22 (corresponding to theanalog comparators 106A and 106B of FIG. 1), and applies a binary resultto the counter circuit 23 (corresponding to the digital counters 107Aand 107B of FIG. 1). The comparator 22 includes an auto zero circuitusing a PSET signal 33 as a trigger, and causes an offset which is alevel difference between the pixel signal 25 and the reference signal 32to be zero. Basically, the offset is caused to be zero, but apredetermined small offset may be applied so that the reference signalintersects with the pixel signal and an output of the comparator 22 isinverted.

The counter circuit 23 receives a counter control signal 34, a counterreset, a counter clock, and a count direction from the timing controlcircuit 31, and performs a counting operation. The counter clock ismasked by the output of the comparator 22, and a digital value accordingto the level of the pixel signal 25 is obtained.

The reference signal generator 35 (corresponding to the reference signalgenerator 105A and the amplifier 105B of FIG. 1) generates the referencesignal 32 that monotonically decreases at the time of measurement.

The bus buffer 24 controls an output, and outputs a value to ahorizontal output line 38 according to a selection signal of the columnscanning circuit 37. A bus buffer with a latch is desirable since an ADconversion operation and a transfer operation can be performed inparallel, but a latch function is not essential. In this embodiment, thebus buffer 24 is described as the bus buffer with the latch. The busbuffer with the latch is different in purpose from a memory which is anessential element in a time division scheme of a related art. The busbuffer 24 is provided to cause an AD conversion operation and aconversion value output operation to be performed in parallel regardlessof an intrinsic operation of AD conversion, and outputs an AD conversionvalue of a previous row at the time of AD conversion of a current row.

The digital compositing unit 39 simultaneously receives the values fromthe horizontal output lines 38 of the two systems, performs a digitalcompositing process on the received values, and outputs the processresult to an external output line 40.

The timing control circuit 31 controls an overall operation sequence ofthe CMOS image sensor using a master clock MCK as a reference of anoperation timing.

[Configuration Example of Unit Pixel]

FIG. 13 illustrates a configuration example of the unit pixel 11. Forexample, the unit pixel 11 includes a photo-diode 111 as a photoelectricconverting element. The unit pixel 11 includes four transistors, thatis, a transfer transistor 112 serving as a transfer element, a resettransistor 113 serving as a reset element, an amplifying transistor 114,and a selecting transistor 115 as active elements with respect to onephoto-diode 111.

The photo-diode 111 performs photoelectric conversion of convertingincident light into charges (here, electrons) of an amount according toa light quantity thereof. The transfer transistor 112 is connectedbetween the photo-diode 111 and a floating diffusion FD serving as anoutput node. The transfer transistor 112 transfers the electrons whichhave been subjected to the photoelectric conversion by the photo-diode111 serving as the photoelectric converting element when a drivingsignal TG is applied to a gate (a transfer gate) of the transfertransistor 112 through the transfer control line LTx.

The reset transistor 113 is connected between a power line VLDD and thefloating diffusion FD. The reset transistor 113 resets potential of thefloating diffusion FD to potential of the power line LVDD when a resetRST is applied to a gate of the reset transistor 113 through a resetcontrol line LRST.

A gate of the amplifying transistor 114 is connected to the floatingdiffusion FD. The amplifying transistor 114 is connected to verticalsignal line 12 through a selecting transistor 115, and configures asource follower together with a constant current source outside thepixel. Further, a control signal (an address signal or a selectingsignal) SEL is applied to a gate of the selecting transistor 115 throughthe selection control line LSEL to turn on the selecting transistor 115.

When the selecting transistor 115 is turned on, the amplifyingtransistor 114 amplifies the potential of the floating diffusion FD, andoutputs a voltage according to the potential to the vertical signal line12. The voltage (analog pixel signal) output from each pixel is outputto the column AD unit 20 (see FIG. 12) through the vertical signal line12. In this implementation example, the multiplexer 21 is used betweenthe vertical signal line 12 and the column AD unit 20.

For example, since the control lines are connected to the gates of thetransfer transistor 112, the reset transistor 113, and the selectingtransistor 115 in units of rows, the above-described operation issimultaneously performed in parallel on pixels of one row.

The reset control line LRST, the transfer control line LTx, and theselection control line LSEL wired in the unit pixel unit 11 are wired asa set in units of rows in a pixel array. The reset control line LRST,the transfer control line LTx, and the selection control line LSEL aredriven by the row scanning circuit 36 serving as a pixel driving unit.

[Configuration Example of Comparator]

FIG. 14 illustrates a configuration example of the comparator 22. Thecomparator 22 includes a differential comparator having a differentialamplifier as a basic configuration. Referring to FIG. 14, thedifferential amplifier includes a pair of input transistors 311 and 312of Nch. whose sources are connected commonly, a pair of transistors 313and 314 of Pch. which are connected between the gates of the pair ofinput transistors 311 and 312 and a power source VDD and include gateswhich are commonly connected, and a current source transistor 315 ofNch. which is connected between a source common connection node of thepair of input transistors 311 and 312 and a ground.

In the differential amplifier, transistors 316 and 317 of Pch. areconnected between the gates and the drains of the pair of inputtransistors 311 and 312, respectively. The transistors 316 and 317function as reset units which are turned on when a reset pulse PSET of alow active is applied to a gate thereof, cause the gates and the drainsof the pair of input transistors 311 and 312 to be short-circuited, andreset capacitors 318 and 319 so that each of voltages of the gates ofthe pair of transistors 311 and 312, that is, each of voltages of twoinput terminals of the comparator 22, has an inverted level of anoutput. In this way, an operation range which will be described latercan be adjusted. In addition, by causing the transistors 316 and 317 todiffer in the size, an inverted level of an output to be reset maydeviate somewhat. This is known as a technique of inverting the outputof the comparator 22 during the AD conversion operation.

The gates of the pair of input transistors 311 and 312 are connected toone terminals of the capacitors 318 and 319, respectively. Thecapacitors 318 and 319 give an offset to an input voltage to adjust anoperation range which will be described later. The other terminal of thecapacitor 318 is connected to the column signal line 12 through which ananalog pixel signal Vx output from each unit pixel 11 of the pixel arrayunit 10 is transferred. The other terminal of the capacitor 319 isconnected to a reference signal line 32 through which an analogreference signal Vref generated by the reference signal generator 35 istransferred.

In the comparator 22 illustrated in FIG. 14, the reset pulse PSET isactivated (a low active) immediately before a comparison of the analogpixel signal Vx and the analog reference signal Vref starts. As aresult, the transistors 316 and 317 are turned on, the gates and thedrains of the pair of input transistors 311 and 312 are short-circuited,and operation points of the pair of input transistors 311 and 312 arereset to a drain voltage.

At this operation point, an offset component of two input terminalvoltages of the differential amplifiers, that is, an offset component ofgate voltages of the pair of input transistors 311 and 312 (a DC offsetof the analog pixel signal Vx and the analog reference voltage Vref andan offset caused by a difference in the threshold value between the pairof input transistors 311 and 312) are almost canceled. In other words,the two input terminal voltages of the differential amplifiers becomealmost the same as each other (an auto zero operation).

Through this auto zero operation, the operation range is adjusted sothat a comparator configured with a transistor is operated at aneffectively operable voltage level when a comparison of the analog pixelsignal Vx and the analog reference signal Vref is performed. Thecomparator 22 outputs a binary result (out2) representing a magnituderelation between the analog pixel signal Vx and the analog referencesignal Vref.

The flow of the AD conversion operation controlled by the timing controlcircuit 31 will be described with reference to a flowchart of FIG. 15.FIGS. 16 and 17 are waveform charts illustrating an example of an ADconversion operation obtained by an operation of the timing controlcircuit 31. FIG. 16 illustrates an example in which input brightness islow and Vx is small, and FIG. 17 illustrates an example in which theinput brightness is high and Vx is large. S1 to S5 of FIGS. 16 and 17correspond to steps ST1 to ST5 of FIG. 15, respectively.

“FD reset” represents a state of a signal line for resetting the levelof the floating diffusion FD of the pixel 11. “FD read” represents astate of a signal line for controlling charge transfer to the floatingdiffusion FD of the pixel 11. “Vref-A” represents potential of an analogreference signal 32A. “Vref-B” represents potential of an analogreference signal 32B. “Vx” represents potential of the analog pixelsignal 25.

“Vco-A” represents an output level of the comparator 22-A. “Vco-B”represents an output level of the comparator 22-B. “CK” represents acounter clock (clock pulse) given through the counter control signal 34,and “Updown” represents a counter direction given through the countercontrol signal 34. “Counter-A” represents an internal count value of thecounter circuit 23-A. “Counter-B” represents an internal count value ofthe counter circuit 23-B.

In step ST1, the following operation is performed. In other words, therow scanning circuit 36 selects a predetermined pixel column of thepixel array unit 10, and the multiplexer 21 selects a predeterminedcolumn signal line 12. Further, the level of the floating diffusion FDof the pixel 11 is reset, the level of the analog reference signal 32output from the reference signal generator 35 is reset, and the countdirection of the counter circuit 23 is set to “down.”

Next, in step ST2, the comparator 22 is reset by the PSET signal 33.Next, in step ST3, the counter clock CK is output to the counter clock34 by Kp times, and a P-phase level (the reset level of the pixel 11) ismeasured. In this case, at a point in time at which the analog pixelsignal Vx intersects with the analog reference signal Vref, the counterclock is masked by the output of the comparator 22, and the countercircuit 23 stops its counting operation.

Next, in step ST4, the following operation is performed. In other words,the level of the analog reference signal 32 output from the referencesignal generator 35 is reset. Further, the count direction of thecounter circuit 23 is set to “up.” The analog pixel signal istransferred to the floating diffusion FD of the pixel 11. After thetransfer, a photoelectron reception well is reset.

Next, in step ST5, the counter clock CK is output to the counter circuit23 by Tover times, and a D-phase level is measured. In this case, at apoint in time at which the analog pixel signal Vx intersects with theanalog reference signal Vref, the counter clock is masked by the outputof the comparator 22, and the counter circuit 23 stops its countingoperation.

The result of the count value represents “the D-phase level−the P-phaselevel” by the measurement of the P-phase level based on the countingdown of the counter circuit 23 and the measurement of the D-phase levelbased on the counting up of the counter circuit 23, and represents thedigital pixel signal of each pixel having no variation which has beensubjected the CDS. The following Formula (11) represents a relationbetween AD conversion results Ta_p, Ta_d, Tb_p, and Tb_d and the digitalpixel signal Va and Vb when AD conversion is performed twice for the Pphase and the D phase and the CDS is digitally performed on thedifference in the voltage level as described above.

$\begin{matrix}\left. \begin{matrix}{V_{a}:={T_{a\_ d} - T_{a\_ p}}} \\{V_{b}:={T_{b\_ d} - T_{b\_ p}}}\end{matrix} \right\} & (11)\end{matrix}$

Next, in step ST6, the count value of the counter circuit 23 is outputto the bus buffer 24. In the AD conversion operation controlled by thetiming control circuit 31, an operation of steps ST1 to ST6 describedabove is repeatedly performed.

When the input brightness is low and Vx is small, both of the analogreference signal Vref-A with a small inclination and the analogreference signal Vref-B with a large inclination intersect with theanalog pixel signal Vx when the D-phase level is measured (see FIG. 18).Thus, in this case, the counter circuits 23A and 23B differ in aquantization step width, but the count values thereof represent validdigital pixel signals.

On the other hand, when the input brightness is high and Vx is large,the analog reference signal Vref-B with a large inclination intersectswith the analog pixel signal Vx when the D-phase level is measured (seeFIG. 19). Thus, in this case, the count value of the counter circuit 23Brepresents the effective digital pixel signal.

FIG. 20 illustrates waveforms of the analog reference signal Vref-A andthe analog reference signal Vref-B. Here, particularly, FIG. 20illustrates a waveform when a column AD converter is employed. Avertical axis (voltage) represents an analog output voltage of a pixel.Here, an average voltage in the light-shielding state (that is, in thestate in which the received light quantity is 0) is represented byVblack, and a voltage of a white level representing a maximum receivedlight quantity is represented by Vwhite.

Further, a horizontal axis (time) represents a time which is a valuecounted by a digital counter. In the case of the column AD converter,the distance between the reference signal generator 35 and thecomparator 22 of FIG. 12 differs depending on a corresponding column,and thus the waveform of the reference signal 32 undergoes a delayaccording to the distance. Due to this delay, the waveforms of theanalog reference signal Vref-A and the analog reference signal Vref-Bgenerated by the reference signal generator 35 differ according to acolumn.

For example, Sa represents the waveform of the analog reference signalVref-A, a signal waveform of a column closest to the reference signalgenerator 35A is represented by Sa_near, a signal waveform of a columnfarthest from the reference signal generator 35A is represented bySa_far, and a signal waveform of a column positioned at the midpoint ofthe two columns is represented by Sa_mid. In other words, a functionthat converts an analog value of the vertical axis into a digital valueof the horizontal axis differs according to the position of a column. Adelay amount of the analog reference signal Vref-A of the farthestcolumn with respect to the closest column of the analog reference signalVref-A is represented by Ta_delay.

Similarly, Sb represents the waveform of the analog reference signalVref-B, a signal waveform of a column closest to the reference signalgenerator 35B is represented by Sa_near, a signal waveform of a columnfarthest from the reference signal generator 35B is represented bySb_far, and a signal waveform of a column positioned at the midpoint ofthe two columns is represented by Sb_mid. In other words, a functionthat converts an analog value of the vertical axis into a digital valueof the horizontal axis differs according to the position of a column. Adelay amount of the analog reference signal Vref-B of the farthestcolumn with respect to the closest column of the analog reference signalVref-A is represented by Tb_delay.

In addition, immediately after a voltage starts to drop from an upperlimit, a curve-like transient phenomenon is shown. In other words, acurve is shown in a falling portion of a waveform around an originalpoint of FIG. 20, and the curve is noticeably shown in a far columnrather than a close column. The transient phenomenon gets weakergradually, a non-steady phenomenon gets stronger gradually, and so astable state is made. Thus, a portion in which the non-steady phenomenonis strong is used in AD conversion requiring linearity of an IOcharacteristic.

In other words, when the start voltage of the analog reference signalVref-A is set to Va_setback and the start voltage of the analogreference signal Vref-B is set to Vb_setback, a conversion result whichis high in linearity of an IO characteristic can be obtained using arange of [Vblack,Vwhite] which is a value range of a pixel signal, thatis, a portion in which the non-steady phenomenon is strong (a range ofthe stable state of FIG. 20) without using a portion in which thetransient phenomenon is strong (a range of the transient state of FIG.20).

For the sake of simplification of description, in FIG. 20, the level ofVblack is used as the original point of the vertical axis. The level ofVblack may undergo a small voltage variation due to influence of atemperature change and the like. However, since the description will becomplicated, here, the variation in the level of Vblack is notillustrated in FIG. 20. The variations in the levels of Vwhite andVthresh that vary according thereto are not illustrated in FIG. 20. Whenthe variations are considered, for example, a problem may be solvedusing a maximum value of Vwhite when Vwhite varies, a minimum value ofVthresh when Vthresh varies, and a minimum value of Vblack when Vblackvaries.

An inclination ΔSa of the analog reference signal Vref-A, an inclinationΔSb of the analog reference signal Vref-B, and a time (count number)Tover taken for AD conversion, and the like which are illustrated inFIG. 20 are obtained based on a noise characteristic of an analog pixelsignal.

First, as constants representing characteristics of a pixel signal inputto the AD converter, an average voltage of a pixel signal in thelight-shielding state (that is, the received light quantity 0) is Vblackof the following Formulas (14) and (15), a standard deviation of noiseof a pixel signal in the light-shielding state (that is, the receivedlight quantity 0) is σ of Formula (14), and a voltage of a white levelrepresenting a maximum received light quantity is Vwhite of thefollowing Formulas (13) and (15). The number of electrons correspondingto the white level is Ewhite of Formula (15).

Further, a start voltage of the analog reference signal Vref-A isVa_setback of Formula (13), and a start voltage of the analog referencesignal Vref-B is Vb_setback of Formula (13). A delay amount of theanalog reference signal Vref-A of the farthest column with respect tothe closest column is Ta_delay of Formula (13), and a delay amount ofthe analog reference signal Vref-B of the farthest column with respectto the closest column is Tb_delay of Formula (13). The constants areestimate values obtained by arbitrary methods.

Further, as constants related to the image quality, a parameter fordeciding the image quality of a dark image portion is Ka of Formula(14), and a parameter for deciding the image quality of a bright imageportion is Kb of Formula (14). For example, normal values of theparameters Ka and Kb are 1 and 1, respectively. When the values decreasethere is a trade-off in which the image quality is somewhat improved,but a time taken for AD conversion increases, and thus a parameter isdecided according to the user's request.

In the above setting, when all values in a range of [0,Vwhite] areassigned to Vthresh in Formulas (13) and (14) and Tover in the followingFormula (12) is evaluated, Vthresh causing Tover to be minimum isdecided. Tover is a minimum time (count number) taken for AD conversion.When Vthresh causing Tover to be minimum is obtained, ΔSa in Formula(14) representing the inclination of the analog reference signal Vref-A,ΔSb in Formula (14) representing the inclination of the analog referencesignal Vref-B, Ta_over in Formula (13) representing a saturation valueof the AD converting unit 29A, and Tb_over in Formula (13) representinga saturation value of the AD converting unit 29B are dependentlydecided.

Tover obtained as described above is a minimum number of pulses appliedfrom the timing control circuit 31 to the counters of the AD converters29A and 29B. In other words, as the timing control circuit 31 isdesigned to transition to the next operation immediately after pulsesare applied to the counters of the AD converters 29A and 29B by Tovertimes, a useless counting operation can be prevented, and an operationtime can be optimized. In other words, the effects by which powerconsumption is reduced and the operation speed increases are obtained.An actual value of Tover may be set to a slightly large value inconsideration of a variation in a device characteristic. Further, thenoise characteristic is not limited to thermal noise and short noise,and Vthresh causing Tover to be minimum may be obtained in terms of allnoises included in an analog pixel signal such as dark current noise andrandom telegraph noise, similarly to the above description.

$\begin{matrix}{T_{over}:={\max\left( {T_{a\_ over},T_{b\_ over}} \right)}} & (12) \\\left. \begin{matrix}{T_{b\_ over}:={\frac{V_{white} - V_{b\_ setback}}{\Delta\; S_{b}} + T_{b\_ delay}}} \\{T_{a\_ over}:={\frac{V_{thresh} - V_{a\_ setback}}{\Delta\; S_{a}} + T_{a\_ delay}}}\end{matrix} \right\} & (13) \\\left. \begin{matrix}{{\Delta\; S_{a}}:={K_{a} \cdot \sigma}} \\{{\Delta\; S_{b}}:={K_{b} \cdot \sqrt{\sigma^{2} + {G \cdot \left( {V_{thresh} - V_{black}} \right)}}}}\end{matrix} \right\} & (14) \\{G:=\frac{V_{white} - V_{black}}{E_{white} - 0}} & (15)\end{matrix}$

FIG. 21 is a waveform chart generated by the timing control circuit 31that controls an operation of the CMOS image sensor 100. VS is avertical synchronous signal representing a one-frame period of time.Basically, Vs represents a timing at which the pixel array unit 10 ofthe CMOS image sensor 100 performs processing in units of images. HS isa horizontal synchronous signal representing one horizontal scanningperiod of time. Basically, HS represents a timing at which the pixelarray unit 10 of the CMOS image sensor 100 performs processing in unitsof rows.

Basically, processing is performed in units of rows, but the CMOS imagesensor 100 performs processing such that each row is divided into aneven-numbered column and an odd-numbered column using the multiplexer21. In this regard, a first row to an n^(th) row are sequentiallyselected at each timing of HS, and an even-numbered column and anodd-numbered column are sequentially selected in each row. Specifically,the row scanning circuit 36 selects a row through the row signal line13, and the multiplexer 21 selects the column signal line 12. In thewaveform chart, using a terminal name of the multiplexer 21, aneven-numbered column is referred to as an “a column,” and anodd-numbered column is referred to as a “b column.”

The AD converter 29 performs the AD conversion operation in response toeach horizontal synchronous signal HS. An output of the result obtainedby the AD converter 29 is controlled by the bus buffer 24, and theresult is sequentially output using the horizontal output line 38. Inother words, as the bus buffer 24 is sequentially scanned by the columnscanning circuit 37, the count value of the counter circuit 23 istransferred to the digital compositing unit 39 through the horizontaloutput line 38. The bus buffer 24 with the latch has an advantage thatthe AD conversion operation and the transfer operation can be performedin parallel and thus the high-speed transfer can be performed, but alatch function is not essential.

Particularly, the present technology is suitable for a configuration inwhich the AD converter of the column AD type illustrated in FIG. 22 ismounted and performs digital CDS. A signal level Vx of a pixelrepresents a reset level during a period of time of a P phase, andrepresents a signal level during a period of time of a D phase. When theP-phase level and the D-phase level are measured, the levels aremeasured by simultaneously comparing the signal level Vx with the twotypes of reference signals Vref-A and Vref-B. A counter-A and acounter-B perform counting in a negative direction in the P phase,perform counting in a positive direction in the D phase, and measure alevel difference. In other words, digital CDS is performed by each ofthe counter-A and the counter-B. In other words, the present technologyis implemented simply using the two counters.

Meanwhile, FIG. 23 illustrates an implementation example of aconfiguration in which the AD converter of the column AD type is mountedand performs digital CDS in order to compare the present technology witha related art. In the related art, in order to differentiate theresolution through time division, after the reference signal Vref-B isused in the P phase, a comparison is made through time division usingthe reference signal Vref-A, and then after the reference signal Vref-Ais used in the D phase, a comparison is made through time division usingthe reference signal Vref-B. The counter is a counter that counts a timein the AD converter. In the related art, since it is necessary toperform a temporary retreat of a value when the resolution isdifferentiated through time division, line memories such as a linememory-A and a line memory-B which are components of the related art arenecessary, and thus the size of a circuit increases. In terms of anoperation, loading and storing of a value with the line memories isperformed, and thus there is a disadvantage in that control iscomplicated.

Further, it is necessary to store an analog level adjusted by the autozero of the comparator. Thus, an analog storage circuit (analog linememory) is necessary, and thus there is a disadvantage in that thecircuit size increases. In addition, since the resolution isdifferentiated through time division, the related art is not desirablein terms of high-speed performance. In other words, the presenttechnology is small in the circuit size, faster in speed, and moreexcellent than the related art. The effect of low power consumption isobtained by this feature.

[Color Array of Pixel Array Unit]

A color array example of the pixel array unit 101 (the pixel array unit10) will be described. For example, the pixel array unit 101 has a colorarray of a Bayer array illustrated in FIG. 24. In this embodiment, theanalog pixel signal Vx is converted into the digital pixel signal usingthe two analog reference signals Vref-A and Vref-B which differ in theinclination, and thus the present technology can be suitably appliedeven to a color array in which pixels greatly differing in spectralsensitivity are mixed as in the following (1) and (2). In other words, asignal of a pixel needing a dynamic range larger than RGB of the relatedart can be converted in a large dynamic range having small noiseregardless of whether a signal level is small or large. In short, in therelated art, ISO sensitivity is changed before capturing is performed,but in the AD conversion method according to the present technology, itis not necessary to change the ISO sensitivity before capturing, andpixels greatly differing in spectral sensitivity may be mixed.

(1) A color array of the pixel array unit 101 includes pixels thatgreatly differ in sensitivity of light such as visible light andinvisible light. For example, FIG. 25( a) illustrates a color array ofthis example, and pixels of visible light (R, G, and B) and pixels ofinvisible light (IR) are present together. FIG. 25( b) illustratesspectral transmittance of a color filter of a pixel, a horizontal axisrepresents a wavelength of light, and a vertical axis representsspectral transmittance. In this example, sensitivity of a pixel ofinvisible light (IR) is better than sensitivity of a pixel of visiblelight (R, G, and B). In this case, for example, it is possible to mainlyconvert an analog pixel signal of a pixel of visible light (R, G, and B)through the AD converting unit 103A having a high gain while mainlyconverting an analog pixel signal of a pixel of invisible light (IR)through the AD converting unit 103B having a low gain.

(2) A color array of the pixel array unit 101 includes pixels whichgreatly differ in sensitivity of light such as color of a narrow bandand color of a broad band. For example, FIG. 26( a) illustrates a colorarray of this example, and a pixel of color (R, G, and B) of a narrowband and a pixel of color (W) of a broad band are presented together.FIG. 26( b) illustrates spectral transmittance of a color filter of apixel, a horizontal axis represents a wavelength of light, and avertical axis represents spectral transmittance. In this example,sensitivity of a pixel of color (W) of a broad band is better thansensitivity of a pixel of color (R, G, and B) of a narrow band. In thiscase, for example, it is possible to mainly convert an analog pixelsignal of a pixel of color (R, G, and B) of a narrow band through the ADconverting unit 103A having a high gain while mainly converting ananalog pixel signal of a pixel of color (W) of a broad band through theAD converting unit 103B having a low gain.

As described above, in the CMOS image sensor 100 illustrated in FIG. 1,a pixel signal output from the unit pixel 11 is converted through thetwo independent AD converting units 103A and 103B using the analogreference signals having different inclinations. In other words, the twoAD converting units that compare a level of an analog reference signalwith a level of an analog pixel signal and convert the analog pixelsignal into a digital pixel signal are arranged, and the digitalcompositing unit 108 simultaneously reads and composites the digitalpixel signals which are outputs of the two AD converting units 103A and103B, and then outputs a composite value. The AD conversion using theanalog reference signals having different inclinations is not performedthrough time division, and a line memory necessary in a time divisionscheme of a related art which temporarily stores the result isunnecessary. Thus, the circuit size is reduced, and the cost of aproduct is reduced. Further, since an AD conversion time can be reduced,a high-speed operation can be performed, and low power consumption canbe achieved.

In addition, in the CMOS image sensor 100 illustrated in FIG. 1, theanalog pixel signal is converted through the two independent ADconverting units 103A and 103B, but pixel signals of two columns of thepixel array unit 101 are selectively supplied to one AD converting unitthrough the multiplexer 102. Thus, an increase in the number of ADconverting units can be suppressed. Thus, even when a mounting area of asemiconductor substrate is narrow, mounting can be easily performed.

In addition, in the CMOS image sensor 100 illustrated in FIG. 1, the ADconverting units 103A and 103B are arranged to be opposite to each otherwith the pixel array unit 101 interposed therebetween. Thus, the two ADconverting units 103A and 103B that operate in parallel can beefficiently arranged on a semiconductor substrate. In other words, evenwhen a column width is narrow, many AD converting units can be arrangedby the opposed arrangement, and thus the mounting area can be reduced.Meanwhile, when a mounting cost need not be considered or when asufficient mounting area is secured, a one-sided arrangement may beemployed. In this case, when the digital compositing units 108 areconfigured in parallel such that one digital compositing unit 108 isarranged in units of columns for each set of the AD converting units103A and 103B, the operation speed of the CMOS image sensor 100 and thedata transfer rate can be increased more.

Further, in the CMOS image sensor 100 illustrated in FIG. 1, theinclinations of the analog reference signals Vref-A and Vref-B in the ADconverting units 103A and 103B are the inclinations ΔSa and ΔSb obtainedbased on the noise characteristic of the pixel signal, respectively.Thus, the reference signal generator can be configured with a simpleintegrator and simply implemented.

Further, in the CMOS image sensor 100 illustrated in FIG. 1, theinclinations of the analog reference signals Vref-A and Vref-B in the ADconverting units 103A and 103B are in the mutually scaled relation.Thus, one analog reference signal generator can be configured with anamplifier or an attenuator, and thus the product cost can be reduced.

Furthermore, in the CMOS image sensor 100 illustrated in FIG. 1, whenthe number of pulses applied to the counter that measures a conversiontime in the AD converting units 103A and 103B reaches a predeterminednumber Tover, the AD converting units 103A and 103B transition to thenext operation. Thus, the operation can be performed at a high speed,and the power consumption can be reduced.

<2. Modified Example>

In the above embodiment, the two AD converting units are arranged inparallel, but a CMOS image sensor in which three or more AD convertingunits are arranged in parallel can be similarly configured, and adynamic range of a digital pixel signal can be further increased. Inother words, the present technology can be successfully applied even toa solid state imaging device having a large saturated electron amount.

Additionally, the present technology may also be configured as below.

-   (1) A solid state imaging device, including:

a first analog-to-digital converting unit that compares a level of ananalog reference signal with a first inclination with a level of ananalog pixel signal output from a pixel array unit, and converts theanalog pixel signal into a first digital pixel signal;

a second analog-to-digital converting unit that compares a level of ananalog reference signal with a second inclination that is different ininclination from the analog reference signal with the first inclinationwith a level of the analog pixel signal, and converts the analog pixelsignal into a second digital pixel signal; and

a digital compositing unit that composites the first digital pixelsignal with the second digital pixel signal, and outputs a compositeresult.

-   (2) The solid state imaging device according to (1),

wherein the digital compositing unit includes

a determining unit that, of the first digital pixel signal and thesecond digital pixel signal, regards a digital pixel signal convertedusing an analog reference signal with a small inclination as one digitalpixel signal, regards a digital pixel signal converted using an analogreference signal with a large inclination as the other digital pixelsignal, compares the one digital pixel signal with a predeterminedthreshold value to determine a magnitude relation, and obtains aselection signal,

a selecting unit that extracts the one digital pixel signal when it isdetermined that the one digital pixel signal is less than thepredetermined threshold value based on the selection signal, andextracts the other digital pixel signal when it is determined that theone digital pixel signal is equal to or more than the predeterminedthreshold value, and

a combining unit that combines the extracted digital pixel signal withthe selection signal, and outputs a combined signal.

-   (3) The solid state imaging device according to (2), further    including

a signal processing unit that receives an output signal of the digitalcompositing unit, and performs magnification correction on the digitalpixel signal based on the selection signal combined with the digitalpixel signal.

-   (4) The solid state imaging device according to (1),

wherein the digital compositing unit combines the first digital pixelsignal and the second digital pixel signal to be composited into asignal with a large bit width and outputs a composite result, orinterleaves and composites the first digital pixel signal and the seconddigital pixel signal, and outputs a composite result.

-   (5) The solid state imaging device according to (4), further    including

a signal processing unit that processes an output signal of the digitalcompositing unit,

wherein the signal processing unit includes

a determining unit that, of the first digital pixel signal and thesecond digital pixel signal, regards a digital pixel signal convertedusing an analog reference signal with a small inclination as one digitalpixel signal, regards a digital pixel signal converted using an analogreference signal with a large inclination as the other digital pixelsignal, compares the one digital pixel signal with a predeterminedthreshold value to determine a magnitude relation, and obtains aselection signal,

a selecting unit that extracts the one digital pixel signal when it isdetermined that the one digital pixel signal is less than thepredetermined threshold value based on the selection signal, andextracts the other digital pixel signal when it is determined that theone digital pixel signal is equal to or more than the predeterminedthreshold value, and

a signal processing unit that performs magnification correction on theextracted digital pixel signal based on the selection signal.

-   (6) The solid state imaging device according to (1),

wherein the digital compositing unit further includes an estimating unitthat calculates correction information used to accurately performmagnification correction.

-   (7) The solid state imaging device according to (6),

wherein the estimating unit obtains a correction value used to convert adigital pixel signal in a manner that an inaccurate distribution of adigital pixel signal becomes an accurate distribution.

-   (8) The solid state imaging device according to any one of (1) to    (7),

wherein the first inclination and the second inclination are each aninclination obtained based on a noise characteristic of the pixelsignal.

-   (9) The solid state imaging device according to any one of (1) to    (8),

wherein the analog-to-digital converting unit transitions to a nextoperation when pulses applied to a counter that counts a conversion timein the analog-to-digital converting unit reach a predetermined number.

-   (10) The solid state imaging device according to (8) or (9),

wherein, where a standard deviation of noise when a light quantity is 0is represented by σ (electron), a number of saturated electrons of apixel is represented by Vwhite, and parameters are represented by Ka andKb,

by Vthresh causing Tover of the following Formula (16) to be minimum,the first inclination is given as ΔSa, the second inclination is givenas ΔSb, and the predetermined number in the analog-to-digital convertingunit is given as Tover.

$\begin{matrix}{{{Tover}:={\max\left( {{{V_{thresh}/\Delta}\;{Sa}},{{V_{white}/\Delta}\;{Sb}}} \right)}}\begin{pmatrix}{{\Delta\;{Sa}} = {{Ka}*\sigma}} \\{{\Delta\;{Sb}} = {{Kb}*\sqrt{\sigma^{2} + {Vthresh}}}}\end{pmatrix}} & (16)\end{matrix}$

-   (11) The solid state imaging device according to (8) or (9),

wherein, where a standard deviation of noise when a light quantity is 0is represented by σ (electron), a number of saturated electrons of apixel is represented by Vwhite, parameters are represented by Ka and Kb,a start voltage of the analog reference signal with the firstinclination is represented by Va_setback, a start voltage of the analogreference signal with the second inclination is represented byVb_setback, a delay amount of the analog reference signal with the firstinclination of a farthest column with respect to a closest column isrepresented by Ta_delay, and a delay amount of the analog referencesignal with the second inclination of a farthest column with respect toa closest column is represented by Tb_delay,

by Vthresh causing Tover of the following Formula (17) to be minimum,the first inclination is given as ΔSa, the second inclination is givenas ΔSb, and the predetermined number in the analog-to-digital convertingunit is given as Tover.

$\begin{matrix}{{{Tover}:={\max\left( {{\frac{V_{thresh} - V_{a\_ setback}}{\Delta\; S_{a}} + T_{a\_ delay}},{\frac{V_{white} - V_{b\_ setback}}{\Delta\; S_{b}} + T_{b\_ delay}}} \right)}}\mspace{79mu}\begin{pmatrix}{{\Delta\; S_{a}}:={K_{a} \cdot \sigma}} \\{{\Delta\; S_{b}}:={K_{b} \cdot \sqrt{\sigma^{2} + {G \cdot \left( {V_{thresh} - V_{black}} \right)}}}} \\{G:=\frac{V_{white} - V_{black}}{E_{white} - 0}}\end{pmatrix}} & (17)\end{matrix}$

-   (12) The solid state imaging device according to any one of (1) to    (11),

wherein the analog reference signal with the first inclination and theanalog reference signal with the second inclination are in a mutuallyscaled relation.

-   (13) The solid state imaging device according to (12),

wherein one of the analog reference signal with the first inclinationand the analog reference signal with the second inclination is generatedby a reference signal generator, and the other is obtained in a mannerthat an analog reference signal generated by the reference signalgenerator is amplified by an amplifier or attenuated by an attenuator.

-   (14) The solid state imaging device according to any one of (1) to    (13),

wherein the pixel array unit is a pixel array unit in which pixelshaving greatly different sensitivity of light are present together.

-   (15) The solid state imaging device according to (14),

wherein the pixel array unit includes a pixel having sensitivity tovisible light and a pixel having sensitivity to invisible light orincludes a pixel having sensitivity in a narrow band or a pixel havingsensitivity in a broad band.

-   (16) The solid state imaging device according to any one of (1) to    (15), further including

a multiplexer that selectively supplies the analog-to-digital convertingunit with pixel signals of a plurality of columns of the pixel arrayunit.

-   (17) The solid state imaging device according to any one of (1) to    (16),

wherein the first analog-to-digital converting unit and the secondanalog-to-digital converting unit are arranged to be opposite to eachother with the pixel array unit interposed therebetween.

-   (18) A method of outputting an imaging signal, including:

comparing a level of an analog reference signal with a first inclinationwith a level of an analog pixel signal output from a pixel array unit,and performing conversion into a first digital pixel signal;

comparing a level of an analog reference signal with a secondinclination which is different in inclination from the analog referencesignal with the first inclination with a level of the analog pixelsignal, and performing conversion into a second digital pixel signal;and

compositing the first digital pixel signal with the second digital pixelsignal and outputting a composite result.

-   (19) An electronic device including a solid state imaging device    including:

a first analog-to-digital converting unit that compares a level of ananalog reference signal with a first inclination with a level of ananalog pixel signal output from a pixel array unit, and performsconversion into a first digital pixel signal;

a second analog-to-digital converting unit that compares a level of ananalog reference signal with a second inclination that is different ininclination from the analog reference signal with the first inclinationwith a level of the analog pixel signal, and performs conversion into asecond digital pixel signal; and

a digital compositing unit that composites the first digital pixelsignal with the second digital pixel signal, and outputs a compositeresult.

-   (20) A solid state imaging device, including:

a plurality of analog-to-digital converting units that compare levels ofa plurality of analog reference signals having different inclinationswith a level of an analog pixel signal output from a pixel array unit,and convert the analog pixel signal into a plurality of digital pixelsignals; and

a digital compositing unit that composites the plurality of digitalpixel signals and outputs a composite result.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

The present disclosure contains subject matter related to that disclosedin Japanese Priority Patent Application JP 2012-072656 filed in theJapan Patent Office on Mar. 28, 2012, the entire content of which ishereby incorporated by reference.

What is claimed is:
 1. A solid state imaging device, comprising: a firstanalog-to-digital converting unit that compares a level of an analogreference signal with a first inclination with a level of an analogpixel signal output from a pixel array unit, and converts the analogpixel signal into a first digital pixel signal; a secondanalog-to-digital converting unit that compares a level of an analogreference signal with a second inclination that is different ininclination from the analog reference signal with the first inclinationwith a level of the analog pixel signal, and converts the analog pixelsignal into a second digital pixel signal; and a digital compositingunit that composites the first digital pixel signal with the seconddigital pixel signal, and outputs a composite result; wherein thedigital compositing unit further includes an estimating unit thatcalculates correction information used to accurately performmagnification correction.
 2. The solid state imaging device according toclaim 1, wherein the digital compositing unit includes a determiningunit that, of the first digital pixel signal and the second digitalpixel signal, regards a digital pixel signal converted using an analogreference signal with a small inclination as one digital pixel signal,regards a digital pixel signal converted using an analog referencesignal with a large inclination as the other digital pixel signal,compares the one digital pixel signal with a predetermined thresholdvalue to determine a magnitude relation, and obtains a selection signal,a selecting unit that extracts the one digital pixel signal when it isdetermined that the one digital pixel signal is less than thepredetermined threshold value based on the selection signal, andextracts the other digital pixel signal when it is determined that theone digital pixel signal is equal to or more than the predeterminedthreshold value, and a combining unit that combines the extracteddigital pixel signal with the selection signal, and outputs a combinedsignal.
 3. The solid state imaging device according to claim 2, furthercomprising a signal processing unit that receives an output signal ofthe digital compositing unit, and performs magnification correction onthe digital pixel signal based on the selection signal combined with thedigital pixel signal.
 4. The solid state imaging device according toclaim 1, wherein the digital compositing unit combines the first digitalpixel signal and the second digital pixel signal to be composited into asignal with a large bit width and outputs a composite result, orinterleaves and composites the first digital pixel signal and the seconddigital pixel signal, and outputs a composite result.
 5. The solid stateimaging device according to claim 4, further comprising a signalprocessing unit that processes an output signal of the digitalcompositing unit, wherein the signal processing unit includes adetermining unit that, of the first digital pixel signal and the seconddigital pixel signal, regards a digital pixel signal converted using ananalog reference signal with a small inclination as one digital pixelsignal, regards a digital pixel signal converted using an analogreference signal with a large inclination as the other digital pixelsignal, compares the one digital pixel signal with a predeterminedthreshold value to determine a magnitude relation, and obtains aselection signal, a selecting unit that extracts the one digital pixelsignal when it is determined that the one digital pixel signal is lessthan the predetermined threshold value based on the selection signal,and extracts the other digital pixel signal when it is determined thatthe one digital pixel signal is equal to or more than the predeterminedthreshold value, and a signal processing unit that performsmagnification correction on the extracted digital pixel signal based onthe selection signal.
 6. The solid state imaging device according toclaim 1, wherein the estimating unit obtains a correction value used toconvert a digital pixel signal in a manner that an inaccuratedistribution of a digital pixel signal becomes an accurate distribution.7. The solid state imaging device according to claim 1, wherein thefirst inclination and the second inclination are each an inclinationobtained based on a noise characteristic of the pixel signal.
 8. Thesolid state imaging device according to claim 7, wherein, where astandard deviation of noise when a light quantity is 0 is represented byσ (electron), a number of saturated electrons of a pixel is representedby Vwhite, and parameters are represented by Ka and Kb, by Vthreshcausing Tover of the following Formula (1) to be minimum, the firstinclination is given as ΔSa, the second inclination is given as ΔSb, andthe predetermined number in the analog-to-digital converting unit isgiven as Tover. $\begin{matrix}{{{Tover}:={\max\left( {{{V_{thresh}/\Delta}\;{Sa}},{{V_{white}/\Delta}\;{Sb}}} \right)}}{\begin{pmatrix}{{\Delta\;{Sa}} = {{Ka}*\sigma}} \\{{\Delta\;{Sb}} = {{Kb}*\sqrt{\sigma^{2} + {Vthresh}}}}\end{pmatrix}.}} & (1)\end{matrix}$
 9. The solid state imaging device according to claim 7,wherein, where a standard deviation of noise when a light quantity is 0is represented by σ (electron), a number of saturated electrons of apixel is represented by Vwhite, parameters are represented by Ka and Kb,a start voltage of the analog reference signal with the firstinclination is represented by Va_setback, a start voltage of the analogreference signal with the second inclination is represented byVb_setback, a delay amount of the analog reference signal with the firstinclination of a farthest column with respect to a closest column isrepresented by Ta_delay, and a delay amount of the analog referencesignal with the second inclination of a farthest column with respect toa closest column is represented by Tb_delay, by Vthresh causing Tover ofthe following Formula (2) to be minimum, the first inclination is givenas ΔSa, the second inclination is given as ΔSb, and the predeterminednumber in the analog-to-digital converting unit is given as Tover.$\begin{matrix}{{{Tover}:={\max\left( {{\frac{V_{thresh} - V_{a\_ setback}}{\Delta\; S_{a}} + T_{a\_ delay}},{\frac{V_{white} - V_{b\_ setback}}{\Delta\; S_{b}} + T_{b\_ delay}}} \right)}}\mspace{79mu}{\begin{pmatrix}{{\Delta\; S_{a}}:={K_{a} \cdot \sigma}} \\{{\Delta\; S_{b}}:={K_{b} \cdot \sqrt{\sigma^{2} + {G \cdot \left( {V_{thresh} - V_{black}} \right)}}}} \\{G:=\frac{V_{white} - V_{black}}{E_{white} - 0}}\end{pmatrix}.}} & (2)\end{matrix}$
 10. The solid state imaging device according to claim 1,wherein the analog-to-digital converting unit transitions to a nextoperation when pulses applied to a counter that counts a conversion timein the analog-to-digital converting unit reach a predetermined number.11. The solid state imaging device according to claim 1, wherein theanalog reference signal with the first inclination and the analogreference signal with the second inclination are in a mutually scaledrelation.
 12. The solid state imaging device according to claim 11,wherein one of the analog reference signal with the first inclinationand the analog reference signal with the second inclination is generatedby a reference signal generator, and the other is obtained in a mannerthat an analog reference signal generated by the reference signalgenerator is amplified by an amplifier or attenuated by an attenuator.13. The solid state imaging device according to claim 1, wherein thepixel array unit is a pixel array unit in which pixels having greatlydifferent sensitivity of light are present together.
 14. The solid stateimaging device according to claim 13, wherein the pixel array unitincludes a pixel having sensitivity to visible light and a pixel havingsensitivity to invisible light or includes a pixel having sensitivity ina narrow band or a pixel having sensitivity in a broad band.
 15. Thesolid state imaging device according to claim 1, further comprising amultiplexer that selectively supplies the analog-to-digital convertingunit with pixel signals of a plurality of columns of the pixel arrayunit.
 16. The solid state imaging device according to claim 1, whereinthe first analog-to-digital converting unit and the secondanalog-to-digital converting unit are arranged to be opposite to eachother with the pixel array unit interposed therebetween.
 17. A method ofoutputting an imaging signal, comprising: comparing a level of an analogreference signal with a first inclination with a level of an analogpixel signal output from a pixel array unit, and performing conversioninto a first digital pixel signal; comparing a level of an analogreference signal with a second inclination which is different ininclination from the analog reference signal with the first inclinationwith a level of the analog pixel signal, and performing conversion intoa second digital pixel signal; and compositing the first digital pixelsignal with the second digital pixel signal and outputting a compositeresult; wherein the compositing includes calculating correctioninformation to accurately perform magnification correction.
 18. Anelectronic device including a solid state imaging device comprising: afirst analog-to-digital converting unit that compares a level of ananalog reference signal with a first inclination with a level of ananalog pixel signal output from a pixel array unit, and performsconversion into a first digital pixel signal; a second analog-to-digitalconverting unit that compares a level of an analog reference signal witha second inclination that is different in inclination from the analogreference signal with the first inclination with a level of the analogpixel signal, and performs conversion into a second digital pixelsignal; and a digital compositing unit that composites the first digitalpixel signal with the second digital pixel signal, and outputs acomposite result; wherein the digital compositing unit further includesan estimating unit that calculates correction information used toaccurately perform magnification correction.
 19. A solid state imagingdevice, comprising: a plurality of analog-to-digital converting unitsthat compare levels of a plurality of analog reference signals havingdifferent inclinations with a level of an analog pixel signal outputfrom a pixel array unit, and convert the analog pixel signal into aplurality of digital pixel signals; and a digital compositing unit thatcomposites the plurality of digital pixel signals and outputs acomposite result; wherein the digital compositing unit further includesan estimating unit that calculates correction information used toaccurately perform magnification correction.